Method of making electrostatic screens,especially for toroidal transformers



ALLY

- March 31; 1970 B. M. FALLER METHOD OF MAKING ELECTROSTATIC SCREENS,ESPECI FOR TOROIDAL TRANSFORMERS Filed March 15, 1967 I nvenlor m QMttornefi United States Patent 3,503,126 METHOD OF MAKING ELECTROSTATICSCREENS, ESPECIALLY FOR TOROIDAL TRANSFORMERS Brian Michael Faller,Hertfordshire, England, assignor to Hawker Siddeley Dynamics Limited,Hatfield, Hertfordshire, England Filed Mar. 15, 1967, Ser. No. 623,327Claims priority, application Great Britain, Mar. 17, 1966, 1 1,827 66Int. Cl. .Hlllf 7/ 06' US. Cl. 29-602 6 Claims ABSTRACT OF THEDISCLOSURE A technique is described for making an electrostatic screenin the form of an open-circuited turn, between two windings laid oneupon another on a toroidal transformer. In this technique, a metallicpaint coating is applied over the end winding, leaving a peripheral gapall around the toroid ring. A strip of insulating tape-is then appliedso that it overlaps one margin only of the applied metallic coating andextends part way only across the width of the gap. After this, thescreen is completed by continuing the paint coating across the outersurface of the insulating tape strip, so that this further applicationof conductive paint units ,with the margin of the previous conductivecoating not covered by the tape, and overlaps but does not touch themargin of the previous paint coating that is covered by the tape.

DESCRIPTION OF INVENTION This invention relates to electrostaticscreens, particularly but not exclusively for toroidal transformers.

The usual method of making an electrostatic screen between two windingson a toroidal core in order to reduce capacitive coupling between thewindings, is to. wind a single close wound layer of wire around thecore, with the start and finish of this' winding open-circuited. Thissuffers from the disadvantages that the screen is far from perfecteither due to the gaps between turns on the outer periphery of thewinding if the wire is close wound on the inner periphery, or there is alarge build-upof wire on the inner periphery if the wire is close woundon the outer periphery.

Also, in order to reduce the volume of the screen, a fine gauge of wiremust be used. This leads to highvoltages being generated between theends of the winding due to the large number of turns required to coverone layer, and these voltages lead to high capacitance currents whichtend to nullify the prime purpose of the screen. Also, where two or morescreens are required using this method, the voltages generated by eachscreen should be equal if the screens are to be effective. To achievethis, each screen must be wound with a different gauge of wire since thewinding length, i.e. the inner periphery of the winding, is changing dueto the buildup of the windings. To ensure that the screen winding alwaysexactly fits the winding length a very tight control is required on thebuild-up and this is very difiicult to achieve in production due to thenormal manufacturing tolerances on cores, tape and wire.

It is an object of the invention to produce a more efficient screen ofelectrical length very close to one turn and which is also easier toconstruct and occupies less volume than the conventional methods.

According to the present invention, an open-circuited turn suitable asan electrostatic screen is formed by the application of a conductivelayer of, for example metallic paint to a suitable insulating backing.The backing may be a form of insulating tape.

3,503,126 Patented Mar. 31, 1970 reference being had to the accompanyingdiagrammatic drawings. The drawings illustrate the application of theinvention to the production of an electrostatic screen over toroidalwindings on an annular core, as in a toroidal transformer. In thedrawings:

FIGURE 1 is a diagrammatic pictorial view of the completed screenedwinding with layers broken away to show the build-up in detail,

FIGURE 2, 3 and 4 are diagrammatic cross sections illustrating threestages in the build-up of the screen, and

FIGURE 5 is a diagram of an interleaving detail of the screen.

It will be understood that, in the drawings, the thicknesses of thevarious layers are exaggerated for the sake of clarity.

The winding or windings 1, which will be underneath the screen, arecompleted and taped up with a suitable type of tape 2, e.g. cotton(FIGURE 2). The winding is then fully impregnated with a suitablevarnish; this is necessary since the screen, when completed, isvirtually impervious to impregnating varnish.

The impregnated winding is next wound with a single half lap layer of anon-porous conformable type of insulating tape 3, e.g. 0.003" unsinteredetched polytetrafiuorethylene. The entire winding with the exception ofa narrow strip 6 encircling the outer periphery, is then coated with aconductive paint layer 4 (FIGURE 3) which will eventually form thescreen. A suitable metallic paint is Aquadag Silver dispersion 915 inM.I.B.K. A simple method of doing this is to apply a mask consisting ofa narrow strip of pressure sensitive tape 5 to the outer peripherybefore coating the winding with paint. After the paint has dried thetape can be removed leaving a narrow unpainted circle around the outerperiphery.

When the paint film 4 is dry a length of self-adhesive polyester tape 7,e.g. Scotch Boy 56, of a suitable width is applied around the outerperiphery of the winding such that one edge of the tape lies halfwayacross the width of-the encircling gap 6 in the paint film (FIGURES 4and 5). The remainder of the encircling gap 6, which is not covered bythe polyester tape, is pain-ted with the metallic paint, the paint film8 being continued part of the way across the width of the polyester tape7 so that the paint film efi'ectively overlaps the paint film under thetape 7 without causing a short-circuited turn.

The connection to the screen may be made by a suitable conductor held incontact, such as a strip of copper foil 9 (FIGURE I) wound around theouter periphery, or by means of a conductor attached by conductivecement, e.g. Aquadag type 1624.

When the paint has been fully cured, which with the paint mentionedrequires typically 15 minutes at C., the screen is covered by one or twohalf lap layers of a suitable tape 10, e.g. cotton. The winding is thenready for either further screens or the rest of the windings. Wheremultiple screens are being made, the tape interlayers may requireimpregnation with a suitable varnish to prevent sponginess.

The above method results in a very elfective electrostatic screen of lowresistance and very low induced voltage with a maximum thickness at theinner periphery of the toroidal winding of about 0.015". This means thattriple screens are possible with less than one sixteenth of an inchbuild-up even on small toroidal windings.

The method may be employed for all types of electrostatic screeningrequired on toroidal windings, and for single turn windings where veryclose coupling is required. By the use of a suitable permeable metallicpaint, magnetic screening of the completed winding may be achieved.Moreover, the use of the 'invention is not restricted to toroids but maybe extended to screening for other electrical apparatus.

I claim:

1. A method of making an electrostatic screen between two windings inthe form of an open-circuited turn, comprising the steps of applying alayer of insulating material over a winding, coating the insulatingmaterial with an electrically-conducting paint leaving a narrow uncoatedstrip along the whole length of the winding, applying a length ofinsulating tape along the uncoated strip such that one edge of said tapelies within the width of the uncoated strip while the other edgeoverlaps the conductive paint coating, and then painting over with saidelectrically-conducting paint the remaining uncovered portion of saidunpainted strip, the conductive paint coating being continued far enoughacross the width of said length of tape for said paint coating tooverlap the paint coating under said further length of tape, thereby tocomplete an open-circuited turn having only one single break at theoverlap.

2. A method according to claim 1, wherein the narrow uncoated strip ismasked, during the painting of said insulating material layers, with anarrow strip of pressure-sensitive tape which is removed afterwards.

3. A method according to claim 1, wherein said insulating material layercomprises a non-porous conformable type of insulating tape wound in asingle half lap layer, and said paint is a metallic paint.

4. A method according to claim 3, wherein said Winding is first tapedwith a textile or porous tape and impregnated before the non-porousinsulating tape is applied.

5. A method according to claim 1, wherein an electrically-conductivestrip is attached to the paint coating for making electrical connectionto the screen afforded by said coating.

6. A method according to claim 1, comprising the further steps of curingthe paint and applying at least one half lap layer of porous tape overthe paint layer.

References Cited UNITED STATES PATENTS 1,320,980 11/ 19.19 Bowman 336-841,586,889 6/1926 Elmer 29-607 X 3,032,729 5/1962 Fluegel 336-843,063,135 11/1962 Clark 336-84 X 3,143,720 8/1964 Rogers 336-84 X3,149,296 9/1964 Cox 336-84 3,292,127 12/ 1966 Dormaier 336-84 CHARLIET. MOON, Primary Examiner C. E. HALL, Assistant Examiner US. Cl. X.R.174-35; 336-84

